This commit enables NGG culling on all GFX10.3 GPUs by default.

A new debug flag environment variable RADV_DEBUG=nonggc is added to
disable this feature on GPUs where it is enabled by default.

The previous perf test flag RADV_PERFTEST=nggc will not be needed on
GFX10.3 anymore but it can still be used to enable the feature on
GPUs where it isn't on by default.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>